b.rkt

#lang racket

(define (f s)
  (define ls (string->list s))
  (reverse (for/fold ([rle `((,(car ls) . 1))])
                     ([c (cdr ls)])
             (match-define (list (cons v n) previous ...) rle)
             (if (char=? v c)
                 (cons (cons v (add1 n)) previous)
                 (cons (cons c 1) rle)))))

(define (g s)
  (let loop ([ls (string->list s)])
    (match ls
      [(list) empty]
      [(list a _ ...)
       (define-values (left right) (splitf-at ls (curry equal? a)))
       (cons (cons (length left) a)
             (loop right))])))